This invention relates to a power supply for driving electro-acoustical transducers under varying operating loads, and more particularly, it relates to a self resonant power supply for driving such transducers. The use of ultrasonic energy for such applications as cleaning, dispersing, welding, and materials treatment involves conversion of electrical power in the form of sinusoidal voltage and current into mechanical vibration. This is usually accomplished through the use of a transducer comprised of a piezoelectric crystal sandwiched between a metal back plate and a sonic energy impedance transformer or horn. When a sinusoidal voltage drive of approximately the same frequency as the mechanical resonance of the transducer is applied to the crystal, the transducer vibrates longitudinally. Piezoelectric transducers are high-Q devices which are particularly well suited for coupling large amounts of acoustic power into loads at a prescribed frequency with little loss, or internal power dissipation. A high-Q device is one that has the ability to resonate with very little expenditure of power needed to excite it. However, a price is exacted in realizing this advantage; that is, an electro-acoustic transducer is very unstable in the vicinity of its operating frequency.
Various solutions to the stability problem appear in the literature and patent art. These solutions usually encompass feedback control loops that include separate free-running oscillators, or other fixed frequency excitation sources such as power line AC voltages. The object in each case is to provide a way to maintain within tolerance the vibrational power input levels to the transducer. This must be done in view of the transducer's natural tendency to fall out of one condition of longitudinal resonance, as the acoustic load changes, and enter into the others.
In addition to providing regulated input power to the transducer to prevent damage, the power drive circuit must track the changing transducer characteristics and maintain constant the amplitude of vibration of the transducer horn or tip. This additional requirement is necessary when mixing certain types of fluids with critical physical properties, such as viscosity and tendancy of entrained particulates to coagulate. These properties are critically dependent upon the power supply being able to maintain tight control over the tip displacement and frequency of vibration.